This invention relates to the joining of an X-ray tube target assembly. More particularly, it relates to the joining of the graphite disk to the metallic layer portion wherein vanadium, titanium or their alloys are used as the brazing material with a particular heating and cooling profile.
In producing new and improved targets for rotary anode X-ray application, it is not only necessary for the targets to survive a more rigorous environment in the X-ray tube but they also must be able to survive more rigorous manufacturing processes such as the vacuum firing at temperatures up to 1800.degree. C. This requires that the improved braze provide the following benefits:
1. The braze melt temperature be low enough to not adversely affect the properties of the TZM alloy which can lose significant strength at about 2000.degree. C.
2. After brazing, have a sufficiently high remelt temperature to support the bond through the additional manufacturing steps such as vacuum firing to 1800.degree. C.
3. The bond should be strong and should not be degraded by thermal excursions during normal X-ray tube operation up to 1500.degree. C. for extended periods of time.
For a particular braze metal to work under the above parameters, it must have certain inherent properties. The first would be that it does not have a eutectic or peritectic reaction with the TZM target layer. Ultimately, the braze metal should form only a simple binary couple with the molybdenum in the TZM alloy. This allows an increase in the remelt temperature of the braze by diffusing molybdenum into it and eliminates the possibility of forming a brittle intermetallic layer. This braze metal should also form a stable carbide with the graphite disk or a series of carbides to approximate a gradation in the carbon content of the joint. A basic rule in bonding dissimilar materials is that for a bond to occur there must be some intermixing of the elements between the two materials. Also, for this bond to have significant strength, the gradation of intermixing should approximate that of a binary diffusion couple where the two materials diffuse together in equal portions.
It has been found that vanadium and titanium form simple isomorphic structures with molybdenum at freezing and both form thermodynamically stable carbides with graphite.
Both vanadium and titanium form carbides with graphite by the eutectic reaction. This is both a benefit to the braze process and an obstacle. First, it is a benefit in that it allows carbon to extend into the braze joint, giving a more gradual gradation of carbon at the interface. It is an obstacle in that the V,Ti-C eutectic reaction temperature is lower than the melt temperature of these metals. To limit the extent of carbon alloying in the braze metal, it is necessary to heat as quickly as possible through the eutectic temperature to the braze metal melt temperature. This allows the braze metal to melt and mix with the TZM alloy while at the same time limiting the eutectic reaction and subsequent alloying between the graphite and the braze metal. A further explanation of a resulting braze joint is given in the diagrams shown in FIGS. 1A-C where the dashed line on the left represents the carbon concentration at the braze interface with the source being the graphite and the dashed line on the right represents the molybdenum concentration at the braze interface with the source being the TZM.
FIG. 1A shows a braze joint that has equal mixing with each of the materials to be bonded. In this case, the braze metal not only acts as a bonding agent but also as a barrier to separate the two materials. The melt temperature for this type bond would be just above the braze metal melt temperature. FIG. 1B shows a braze joint that had extensive mixing with carbon but limited mixing with molybdenum. The melt temperature for this joint would be above the eutectic melt temperature with carbon but less than the melt temperature of the braze metal. FIG. 1C shows the braze joint for this invention. It is seen that there is some mixing between carbon and the braze metal but that there is extensive mixing between molybdenum and the braze metal. With this particular type of intermixing of the subject materials, one can achieve all of the stated objectives of the improved braze joint. The brazing temperature is less than 2000.degree. C. for the braze metals, remelt temperature is enhanced by the extensive alloying of molybdenum in the braze joint, and the bond is thermally stable up to 1800.degree. C.
U.S. Pat. No. 4,715,055 describes a technique for using platinum as a braze metal. Experiments have shown that this braze joint is thermally unstable and that temperatures above 1500.degree. C. can quickly degrade the bond strength. Another patent, U.S. Pat. No. 3,710,170, describes another technique whereby a molybdenum-carbon eutectic is formed by heating the assembly to 2200.degree. C. This would degrade the physical strength of the TZM alloy. Also, it has been seen that the Mo.sub.2 C carbide tends to form many cracks which can impair heat flow and joint strength. British Patent 1,383,557, describes a process whereby a braze metal of zirconium or titanium or their alloy is used. This patent states that the assembly should be heated to a temperature just above the target metal substrate-braze metal eutectic temperature but below the melt temperature of the braze metal. It has been determined that there is no eutectic reaction between titanium and molybdenum (or tungsten, or tantalum, or rhenium). Therefore, to use titanium and heat the assembly to a temperature below the melt temperature of titanium means that this bond was formed by the eutectic reaction between titanium and carbon. The remelt temperature would be approximately 1650.degree. C., well below the stated goal of 1800.degree. C. As for zirconium, it does have a eutectic reaction with the target layer, specifically molybdenum. The eutectic reaction temperature is approximately 1575.degree. C. Again, this is well below the stated remelt temperature of 1800.degree. C. Also, a brittle intermetallic layer is formed at the target layer-braze joint interface which also degrades the bond strength.
It is, therefore, an object of the present invention to provide an improved composite X-ray target with a brazed interconnection having improved bond strength and with selected temperature characteristics. Another object of the present invention is to provide an improved method of brazing composite X-ray tube targets with a particular heating and cooling profile which utilizes vanadium or titanium as the brazing material.
These objects and other features and advantages will become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.